Operating environments using threaded fasteners subject to severe shock, vibration and abrupt load changes have challenged the efforts of designers for many years. Under these and the like conditions it is extremely difficult to safeguard against loosening and premature failure of the fastener with attendant serious consequences to the structure and to the safety of persons in the vicinity.
Demountable dual cargo wheel assemblies present particularly frustrating and vexatious problems because of the unusually severe shock, vibration and stress reversal conditions encountered in normal every day usage. These wheels are secured to the hub by a ring of threaded fasteners typically embodying a threaded stud fixed to the hub and brake drum unit and to which fasteners the very heavy wheels are secured. The fastener receiving holes must be appreciably larger than the diameter of the fastener shanks to permit assembly and disassembly of these heavy wheels without special equipment. This essential and mandatory radial clearance permits limited relative rotary movement between the hub and the wheel during both vehicle propelling and braking of the wheels. Likewise, this radial clearance permits limited relative movement between the wheel and the hub while travelling over rough ground, across pot holes and the like. These several and distinctly different types of relative movement cause wear in progressively increasing amounts with usage and results inevitably in the loosening of the fasteners.
Another equally serious cause of fastener loosening is the fact that prior fastener assemblies are so constructed as to pass major portions of the operating stresses through the fastener threads. Such threads unavoidably, are subject to widely varying manufacturing tolerances and the severe ever-changing load stresses must be transmitted across the loose ill-fitting threads. Additionally, the threads deflect under stress thereby augmenting the permissible play and introducing further thread-loosening forces.
A proposal made to counteract several adverse characteristics of dual wheel assemblies are disclosed in Eksergian, U.S. Pat. No. 1, 883,641. In a first embodiment this patentee proposes a collet surrounding the hub studs. The exterior of this stud and the base of the fastener nut are similarly beveled at a taper angle of 1 to 1 to internest with the similarly beveled bores of the fastener mounting holes in the wheel. Additionally, the nave of the wheel disc through which the fastener studs pass is made of resilient material and formed with annular channels facing one another with their radially spaced rims in abutting contact. According to the concept changing load stresses cause these resilient annular sections to flex axially of the fasteners to maintain the fastener threads under high loads irrespective of changing loads encountered in usage. in a second embodiment Eksergian suggests that this resilient mode of maintaining the fastener threads preloaded is sufficient to eliminate the need for his contractable collet. However, neither of these expedients is useful in practice for reasons self-evident from the fact that cargo wheel discs are customarily from one-half to 3/4 of an inch or more in thickness.
Manning, U.S. Pat. No. 2,516,896 proposes a single collet ring assembled concentrically of the hub itself and intended to be contracted against the hub by tightening of the hub fasteners. However, Manning lacks means for eliminating the necessary radial play between the wheel and its mounting studs with the attendant wear unavoidably associated with this play and the cumulative effect of this play on the loosening of the hub nut. Additionally, his stud threads are exposed to severe hazards during mounting and demounting of the wheels.
Putnam, U.S. Pat. No. 1,518,550 recognizes the advantage of using internally threaded studs in combination with cooperating cap screws. However, only a small fraction of the wheel bore seats on the stud and the remainder of the multi-part wheel disc lies beyond the end of the stud. No provision is made for eliminating the relative torsional movement between the hub and the wheel unavoidable because of the necessary clearance between the studs and the stud mounting openings in the wheel.
Another prior attempt to eliminate relative movement between the vehicle wheel and its hub is shown in Hecht, U.S. Pat. No. 1,607,274. His proposal requires mounting one of the dual wheels directly on the brake drum by one set of fasteners and the other wheel directly on the wheel hub itself by stud nuts having tapered segmental parts held loosely captive on the nut. It is impossible to retain but one of the dual wheels to the hub by such a fastener expedient.
Some of the foregoing and other prior proposals endeavor to lock out the very considerable degree of play between the hub studs and the wheel mounting holes using a lug nut having a semi-spherical or steeply tapered inner rim nesting into a similarly shaped socket at the outer end of the mounting bore hole in the wheel. This merely assures that the heavy radial and torsional load forces are transmitted to the hub through the threads of the studs and lug nuts.
Ash U.S. Pat. No. 2,336,767 provides a fastener for securing only a single demountable wheel to the hub using two collets each having a taper angle of 1 to 1. The inevitable result is that this steep angle directs wheel shock and load forces into the hub via the stud and lug nut threads. It is well known that this is a major cause of fastener loosening.
Other U.S. Pat. Nos. showing the extensive work done by numerous designers in attempts to provide a foolproof and shock-proof wheel assembly include Hunt 1,858,827; Michelin 1,889,837; Hunt et al. 1,910,250; LaBrie 1,914,996; Ash 1,939,641; Crowther 1,940,675; Zerk 2,123,130; Eksergian 2,161,740; Eksergian 2,217,646; Ash 2,533,707; Eksergian 2,597,835; Candy et al. 2,636,783; Hedlund et al. 2,877,054; Hykes 2,925,304; and Verdier 3,386,771.